The Trap Guard is a great tool to have in your game, especially if you don't like playing traditional closed guard. It keeps you safe (even from punches) and will set up a ton of attacks and sweeps.
Obviously, once you have such a tight control like this, controlling an arm is easy. In part 3 of the series, he shows how simple it can be to set up the triangle from this position. The transition can feel a little awkward at first, but it's fast and powerful once you drill it. He also shows a lot of grip details that make this simple once you understand the position.